| Summary: | The study of the ortho-to-para ratio of assorted gas-phase interstellar molecules such as H2, H2O, NH3, and H2O+ has gained interest in recent years, based partially on new spectral observations of light hydrides by the Herschel Space Observatory. Although these ratios can yield valuable information about the thermal history of the interstellar cloud where the molecules are found, an understanding of how the ratios are determined involves a number of often poorly studied processes, which can include both gas-phase and grain-surface reactions. In this article, we consider the processes that determine the ortho-to-para ratio of the molecular ion H2O+ in diffuse interstellar clouds and attempt to reproduce an unusual observed ratio for this ion. In addition to the study of ortho-to-para ratios, we look carefully at current uncertainties in the gas-phase formation of large neutral molecules in cold dense interstellar clouds via ion-neutral radiative association and dissociative recombination, among other processes.
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